Some wireless digital communication systems use frequency division multiple access (FDMA) to divide the RF communication spectrum into a plurality of radio channels corresponding to different carrier frequencies. Some wireless communication systems divide the same carrier frequency into a periodic train of time slots that are separately used by RF transmitters, referred to as time-division-multiple-access (TDMA). Example TDMA digital cellular systems include IS-136, GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and PDC (Personal Digital Cellular).
Still some other wireless communication systems use code division multiple access (CDMA) to allow different signals to share the same carrier frequencies. Example CDMA cellular systems include IS-95, cdma2000, and WCDMA (wideband-CDMA). In a CDMA system, an information data stream to be transmitted is impressed upon a higher rate data stream, known as a spreading sequence, to provide a stream of bits referred to as a chip sequence. A receiver then correlates a received chip sequence to the spreading sequence to recover the information data stream.
In RF communications systems, signals that are transmitted to a receiver typically suffer from distortion due to time dispersion, caused by, for example, signal reflections from buildings and other reflective terrestrial surfaces. Multi-path dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver receives many “images” having different and varying delays and amplitudes. Thus, when multi-path time dispersion is present, the receiver receives a composite signal of multiple versions of a transmitted bit that have propagated along different paths (referred to as “signal paths” or “rays”). Each signal path has a certain time of arrival relative to the arrival of a shortest, or first received, signal path. Receivers may collect the signal energy from the different multi-paths to reproduce the transmitted information.
A receiver may model the channel as a tapped delay line, in which tap locations correspond to ray or path delays and the tap coefficients correspond to channel coefficients. The delays and coefficients may be estimated and used to demodulate a received signal. For TDMA, or other narrowband systems, a receiver may include a coherent demodulator, such as a decision feedback equalizer (DFE) or maximum likelihood sequence equalizer (MLSE). For CDMA systems, the demodulator may include a RAKE receiver.
A RAKE receiver may be used to detect individual signal images or versions using correlation operations, to correct for different time delays, and to combine the detected signal images. RAKE receivers include processing elements or “fingers”. The receiver may estimate the delays of the multi-paths and assign a finger to each delay. The finger then despreads the signal image. The finger outputs may be combined by weighting them and adding them together.
Delay estimation may be challenging when the rays are closely spaced relative to the bit period in TDMA systems or the chip period in CDMA systems. Delays may be particularly difficult to estimate when the rays interact with one another, such as due to a ringing of pulses in a CDMA chip sequence, and/or when the delays occur within several bit/chip periods.
One approach to estimating delays in multi-path signals is discussed in the commonly assigned U.S. patent application Ser. No. 09/005,580, filed Jan. 12, 1998, entitled “METHOD AND APPARATUS FOR MULTI-PATH DELAY ESTIMATION IN DIRECT SEQUENCE SPREAD SPECTRUM COMMUNICATION SYSTEMS”, which is incorporated herein by reference.